During the past several years, silicon CCD imaging technology has made many significant advances, until today the technology has capabilities for high quantum efficiency, a high modulation transfer function (MTF) and a high resolution over the entire visible spectrum and down into the vacuum ultraviolet, or approximately from 1000 to 6500 Angstroms. Unfortunately, the performance of the conventional CCD silicon structure degrades rapidly outside of this range. In the case of shorter wavelengths, the degradation is due to photon penetration below the usually shallow depletion region. This photon penetration causes charge "cloud" formation when the photon is absorbed, and this "cloud" spreads to neighboring CCD collection sites. Further efficiency is lost because of the loss of charge due to recombination.
In the case of longer wavelengths, the performance problem is also due to the penetration of photons beneath the depletion region, and the subsequent loss of signal and MTF due to charge spreading. This problem must be solved before CCD imagers can be applied to such applications as X-ray imaging for medical and industrial uses, as well as to commercial and military applications in the near infrared, such as detectors for 1.06-micron laser sources.
Researchers have attempted to solve this problem by providing a silicon CCD structure that can gather charge from a very large silicon volume, while still maintaining the capability for small cells and a minimum need for extreme cooling. In the past several years, certain researchers have built CCD's on very high-resistivity silicon. This approach is however very limited in the depletion depth that can be obtained, and because it is susceptible to a large number of defects. In addition, the very deep depletion depths used in this approach result in very large dark currents and very high fields.
A need has therefore arisen for a high-resolution CCD imager that is capable of resolving both the near infrared and X-ray regions without high fields and large dark currents.